Generally, due to excellent mechanical and thermal properties and high gas-barrier properties, polyamide stretch films obtained by stretching uniaxially or biaxially have been used widely as materials for packaging of liquid type food, moist food, frozen food, retort food, paste type food, livestock meat and seafood products, and heavy products such as soup, konjak, hamburg, miso paste, ham, rice, etc. These films to be used mainly for packaging are required to have good slipperiness and excellent workability for printing, vapor deposition, lamination, and bagmaking. However, conventional polyamide biaxially oriented films have a defective point in that their slipperiness is worsened by moisture absorption and softening in highly humid environments, and consequently, there are various problems attributed to deficiency in slipperiness at the time of handling or processing the films particularly in rainy seasons.
As a means for improving the slipperiness of a polyamide resin film, the following methods are proposed: (1) a method for decreasing the contact surface area of films on one another by adding fine particles of silica, kaolin, or the like to a resin, extruding these fine particles and forming fine projections on the surface of films by a stretching treatment; (2) a method for decreasing the contact surface area of films on one another by forming spherulite in the films; (3) a method for decreasing mutual action in contact portions of films on one another by adding an organic lubricant, such as higher fatty acid bisamide compounds or the like, to a polyamide resin; and (4) a method for making a film having a multi-layer structure by a coating, lamination, or co-extrusion method and forming a layer with improved slipperiness obtained by the methods (1) to (3) as a top surface layer.
However, in order to reliably obtain satisfactory workability even under high humidity for polyamide resin films obtained by methods (1) and (2), it is necessary to increase the addition amount of fine particles or spherulite for forming the projections on the film surface, and owing to that, the transparency of the film is lowered and it becomes a serious defect for packaging use for which a clear appearance is required. As compared with polyamide resin films obtained by methods (1) and (2), the polyamide resin film obtained by method (3) is provided with high slipperiness although slightly inferior in transparency. However, if the addition amount of an organic lubricant is increased to obtain sufficient slipperiness, it results in worsening of the sticking property and wettability to other materials at the time of layering or laminating the film on other materials and undesirably affects the processability for printing, vapor deposition, lamination, etc.
A method is therefore proposed for providing excellent slipperiness in highly humid environments by specifying the size of inorganic fine particles and the pore volume and controlling the ratio of the layer to which the fine particles are added (reference to Patent Document 1). In this method, although having remarkably excellent slipperiness in highly humid environments as well as transparency sufficient for packaging materials without any problem, films obtained by the method are difficult to be used for purposes required to have good designing properties.
As another method, a method is proposed for keeping slipperiness by adding a semi-aromatic polyamide resin to an aliphatic polyamide resin as a main component, making a resin with low moisture absorption exist in the film surface, and suppressing softening of the surface (Japanese Patent No. 3671978).
However, although the transparency decrease is suppressed to improve the slipperiness by addition of a semi-aromatic polyamide resin by this method. While this method can be an excellent technique for controlling the slipperiness, the method has a problem in terms of the cost that the raw material cost of a semi-aromatic polyamide resin is high, and the production cost tends to be high.